ar de Arellano-Lopez et al / Journal of the European Ceramic Society 21(2001)245-250 B1=7.6710x10-10 MPa- s-)and T2=1500c boundary phase 9-24 Several authors have suggested (B2=7.7903x10-9MPa-ls-I)can be used to obtain an that compressive creep occurs by grain-boundary sliding estimate of the activation energy as follows probably accommodated by solution/reprecipitation of the Si3 N4 phase and viscous flow of the boundary e-O/RT1 BI RTI -O/RT2 B It was determined that the polycrystalline Si3N4 phase controls the creep-rate of the parallel-orientation FMs Eq(3) yields Q=570+150 kJ /mol. A specific experi This conclusion is similar to that obtained from creep of ment to determine g directly was performed by tem- unidirectional Sic-fiber/mullite composites. 25 In gen- perature jumps between 1400 and 1500C on a FMpara eral, creep of unidirectional FMs is likely to be domi- sample, under a constant stress of 105 MPa(Fig 8). The nated by the weaker, cell-boundary phase when esult was 0=625+40 kJ/mol, which is in fair agree- compression is perpendicular to the cells and dominated ment with the o value calculated when all of the data by the stronger cells when compression is parallel to the points are considered These values for n and 0, are in excellent agreeme vith previous work on Si3 N4 that contained a grain 4. Conclusions 10×10°「T+TT Compressive creep of a unidirectional Si3 N4/BN fibrous monoliths and its individual constituents. bN 1300-1500%C. When the cells in the fibrous monolith were oriented perpendicular to the stress axis, steady- state deformation was limited to very low stresses. For U0.5×10 this orientation fracture of the bn matrix occurred followed by fracture of the Si3 N4 cells. When the cells in the fibrous monolith were oriented parallel to the stress axis, steady-state deformation and permanent strains to 10% were achieved. In this orientation, plastic deforma- tion was controlled by deformation of the Si3 N4 cell 050100150200 Acknowledgements Fig. 7. CL creep results on a linear-linear graph; O, A=1400C and le thank M. rigali and M. Sutaria for supplying the ●,△=1500°C test materials. This work was supported in Sevilla by the Spanish Ministerio de educacion, CICYT Project MAT97-1007-C02, and at Argonne National Labora tory by the Defense Advanced Research Projects Agency, through a Department of Energy Interagency Agreement, under Contract W-31-109-Eng-38. J. L.R. is grateful to IBERDROLA for providing funds during his stay at the References atent 4, 772, 524, 20 September 1988 as, G. E, Brady, G.A. Somas, S, Bard, A and Zywicki, G. Process for preparing tex- tured ceramic composites. US Patent. 5, 645. 781, 8 July dy, G. A, Abdali, U, Zywicki. G. and Hallora 105/RT cessed ceramics. Mater. Sci. Eng 1995. A195, 263-268 4. Kovar, D, King. B. H. Trice, R. w. and Halloran, J. W. Fig 8. CL test to determine 0=625+40 kJ/mol for FMpara sample Fibrous monolithic ceramics. J. Am. Ceram Soc., 1997. 80. 2471-(B1=7.671010ÿ10 MPaÿ1 sÿ1 ) and T2=1500C (B2=7.790310ÿ9 MPaÿ1 sÿ1 ) can be used to obtain an estimate of the activation energy as follows: eÿQ=RT1 eÿQ=RT2 ˆ B1 B2 ) Q ˆ RT1T2 T1 ÿ T2 ln B1 B2  : …3† Eq. (3) yields Q=570150 kJ/mol. A speci®c experi￾ment to determine Q directly was performed by tem￾perature jumps between 1400 and 1500C on a FMpara sample, under a constant stress of 105 MPa (Fig. 8). The result was Q=62540 kJ/mol, which is in fair agree￾ment with the Q value calculated when all of the data points are considered. These values for n and Q, are in excellent agreement with previous work on Si3N4 that contained a grain￾boundary phase.19ÿ24 Several authors have suggested that compressive creep occurs by grain-boundary sliding probably accommodated by solution/reprecipitation of the Si3N4 phase and viscous ¯ow of the boundary phase.19ÿ24 It was determined that the polycrystalline Si3N4 phase controls the creep-rate of the parallel-orientation FMs. This conclusion is similar to that obtained from creep of unidirectional SiC-®ber/mullite composites.25 In gen￾eral, creep of unidirectional FMs is likely to be domi￾nated by the weaker, cell-boundary phase when compression is perpendicular to the cells and dominated by the stronger cells when compression is parallel to the cells. 4. Conclusions Compressive creep of a unidirectional Si3N4/BN ®brous monoliths and its individual constituents, BN and Si3N4, was investigated in inert atmosphere at 1300±1500C. When the cells in the ®brous monolith were oriented perpendicular to the stress axis, steady￾state deformation was limited to very low stresses. For this orientation, fracture of the BN matrix occurred, followed by fracture of the Si3N4 cells. When the cells in the ®brous monolith were oriented parallel to the stress axis, steady-state deformation and permanent strains to 10% were achieved. In this orientation, plastic deforma￾tion was controlled by deformation of the Si3N4 cells. Acknowledgements We thank M. Rigali and M. Sutaria for supplying the test materials. This work was supported in Sevilla by the Spanish Ministerio de EducacioÂn, CICYT Project MAT97-1007-C02, and at Argonne National Labora￾tory by the Defense Advanced Research Projects Agency, through a Department of Energy Interagency Agreement, under Contract W-31-109-Eng-38. J.L.R. is grateful to IBERDROLA for providing funds during his stay at the University of Sevilla. References 1. Coblenz, W. S. Fibrous monolithic ceramic and method for pro￾duction. US Patent 4,772,524, 20 September 1988. 2. Popovich, D., Halloran, J. W., Hilmas, G. E., Brady, G. A., Somas, S., Bard, A. and Zywicki, G. Process for preparing tex￾tured ceramic composites. US Patent. 5,645,781, 8 July 1997. 3. Hilmas, G., Brady, G. A., Abdali, U., Zywicki, G. and Halloran, J., Fibrous monoliths: non-brittle fracture from powder-pro￾cessed ceramics. Mater. Sci. Eng., 1995, A195, 263±268. 4. Kovar, D., King, B. H., Trice, R. W. and Halloran, J. W., Fibrous monolithic ceramics. J. Am. Ceram. Soc., 1997, 80, 2471± 2487. Fig. 7. CL creep results on a linear±linear graph; *,~=1400C and *,~=1500C. Fig. 8. CL test to determine Q=62540 kJ/mol for FMpara sample; stress=105 MPa. A.R. de Arellano-LoÂpez et al. / Journal of the European Ceramic Society 21 (2001) 245±250 249